An electrophoretic cell is a cell comprised of pigment particles suspended in a fluid and uses electrophoresis to switch between the following two states:
Distributed State:
Particles are positioned to cover the horizontal area of the cell. This can be accomplished, for example, by dispersing the particles throughout the cell, by forcing the particles to form a layer on the horizontal surfaces of the cell, or by some combination of both.
Collected State:
Particles are positioned to minimize their coverage of the horizontal area of the cell, thus allowing light to be transmitted through the cell. This can be accomplished, for example, by compacting the particles in a horizontal area that is much smaller than the horizontal area of the cell, by forcing the particles to form a layer on the vertical surfaces of the cell, or by some combination of both.
The electrophoretic cell can serve as a light valve since the distributed and collected states can be made to have different light absorbing and/or light scattering characteristics. As a result, an electrophoretic cell can be placed in the light path between a light source and a viewer and can be used to regulate the appearance of a pixel in a display. The basic operation of reflective electrophoretic cells along with the examples of various electrode arrangements is described in U.S. Pat. No. 5,745,094.
Reflective color displays are known that use liquid crystals in conjunction with a fixed polarizer element to control the intensity of light reflected from each pixel. Since polarizers absorb the fraction of light whose polarization is not aligned with their active axis, and since this absorption varies with the angle of incidence, displays based on their use suffer from both limited reflectivity and viewing angle.
Other reflective color displays are known that use a solution of a dichroic dye in single or multiple layers of either a nematic or cholesteric liquid crystal material. Using a single nematic layer requires the use of a fixed polarizer element and therefore suffers from the aforementioned limitations. Using one or more cholesteric layers, or more than one nematic layer, eliminates the need for a fixed polarizer element and increases the achievable reflectivity. This approach still relies on the selective absorption of polarized light and, as a result, the contrast changes with viewing angle.
Other reflective color displays are known that use scattering liquid crystal materials, such as polymer-dispersed liquid crystal materials or scattering-mode polymer stabilized cholesteric texture materials, to control the intensity of light reflected from each pixel by switching between a turbid state and a uniform state. Since these materials only weakly scatter light in their turbid state, reflective displays based on them have a low diffuse reflectance and therefore also suffer from low brightness.
Other reflective color displays are known that use reflecting liquid crystal materials, such as reflective-mode polymer-stabilized cholesteric texture materials or holographic-polymer-dispersed liquid crystals, to control the both the intensity and color of reflected light reflected from each pixel via diffraction effects. Since these depend on diffraction effects, it is difficult to simultaneously achieve large viewing angle, high reflectance, and angle independent color.
Reflective color electrophoretic displays have been proposed in the prior art. Japanese Patent No. JP 1267525 assigned to Toyota Jidosha KK describes an electrophoretic display having colored (blue and yellow) particles with different zeta potentials in a solution of red dye to give a multicolored (yellow, green and red) display. When a certain voltage is applied to the pixels, the yellow particles are pulled to the front transparent electrode and the viewer sees yellow. At a higher voltage, the blue particles are also pulled to the front electrode and the viewer sees green. When the particles are pulled off the transparent electrode, the colors of the particles are hidden by the dye solution and the viewer sees red.
Evans, et al., in U.S. Pat. No. 3,612,758, describe a reflective electrophoretic display having pigment particles of a single color in a contrasting dye solution. In this scheme, under the influence of an electric field, the particles migrate to a front transparent electrode and the viewer sees the color of the particles. When the field is reversed, the particles migrate away from the front transparent electrode, are hidden in the dye solution, and the viewer sees the color of the dye solution.
In the two electrophoretic display patents above, color contrast and reflectance depend on the presence or absence of particles at the front window. Since the dye solution can not be completely removed from the space between the particles when they are at the front window, displays based on this approach do not produce highly contrasted images and generally have a low reflectance.
Hou, in WO 94/28202, describes a dispersion for a reflective electrophoretic display comprised of two differently colored particles that are oppositely charged. The polarity of the voltage applied to the cell determines the polarity of the particle attracted to the front transparent electrode, and hence determines the color seen by the viewer. Since the viewer sees either one of two colors, this approach produces monochrome images and therefore has a limited color gamut.
Di Santo, et al., in U.S. Pat. No. 5,276,438, disclose a reflective electrophoretic display in which a mesh screen, disposed behind the front window and covering the viewing area of the display, is used either with or without a dyed suspension fluid to hide particles of a single color from the viewer. When the particles are positioned in front of the mesh, the viewer sees the color of the particles. When the particles are positioned behind the mesh, the viewer sees a mixture of the mesh and particle colors. As a result, the contrast produced by this approach is limited by the open area of the mesh. In addition, this approach produces monochrome images and therefore has a limited color gamut.
There is a continuing need in the art for a low-power reflective color electrophoretic display with high reflectance, high image contrast, and large color gamut. It is therefore an object of the present invention to provide a low-power, reflective color electrophoretic display having improved reflectance, image contrast, and color gamut. Other objects and advantages will become apparent from the following disclosure.